ATR and medulloblastoma: To assess whether one or possibly both of these DDR signalling cascades are activated in the medulloblastoma biopsy specimens, we employed well‐characterized and validated antibodies to activated (phosphorylated) forms of ATM (ATM‐P), Chk2 (Chk2‐P), ATR (ATR‐P) and Chk1 (Chk1‐P) using the same method as for the detection of γH2AX.